This invention can be used in field of remote identification of objects, principally moving objects. For example, shipping containers, railroad cars and automobiles need to be identified while they are in use, often while moving. Systems useful for remote identification using active or passive "tags" which backscatter-modulate a continuous wave, r.f. transmitted signal with a digital identification code have been described in U.S. Pat. No. 4,739,328 and U.S. Pat. Application Ser. No. 885,250, filed July 14, 1986, assigned to the same assignee as this invention. The backscatter-modulated signal is received, usually by the same system which transmitted the original signal, and the digital code is demodulated and decoded, providing identity information of the tagged object, or other information which may be desired.
In many applications of these systems, there are several tagged objects near each other. Each of these tags has the capability of backscatter-modulating the transmitted signal. Often some of these objects are the ones from a desired signal, and others are not. A good example of this is in toll road toll collection lanes. A separate "reader", i.e., a transmitter-receiver, is normally used for each lane. Alternatively, a single transmitter-receiver transmitting multiple, different signals may be used, having separate transmitting and receiving antennas for each lane. The lane-specific antenna or transmitter-receiver must be capable of discriminating between the desired, returned signal from the correct tag after backscatter modulation, and a similar signal which may be returned from a tag on a car in a different lane for which this particular transmitter-receiver or antenna is not intended.
In the past, many different techniques for discrimination have been employed. Antenna location and orientation has been carefully chosen to maximize the amplitude of the return signal from the desired tag location. For example, antennas can be spaced far enough apart to avoid interference. In addition, the receiver sensitivity can be selected to maximize the reception from the desired tag reading area and minimize it from other areas. Various shielding techniques also have been used.
Shielding is an erratic solution because unpredictable signal reflection can elude a shield or deflect a desired signal in an unexpected manner, confusing the signals at the reader. Antenna location and orientation is not always suitable since structures such as toll booths, lane barriers and the like often interfere with ideal locations and spacing of antennas. Morever, the transmitted signals are often not narrow and well defined, making precise control difficult. Adjusting receiver sensitivity is also sometimes erratic because reflected signals can combine with the original or other reflected signals, resulting in an unpredictable amplification of the received signal which fools even the adjusted receiver.
What is desired is to have something in the transmitted signal itself which renders it capable of being discriminated, in its backscatter-modulated form, from other backscatter-modulated return signals from other tags even though all the tags backscatter-modulate the transmitted signal in precisely the same manner.